1. Field of the Invention
This invention pertains in general to the field of surface plasmon resonance (SPR) spectroscopy. In particular, the invention relates to a novel SPR approach involving the coupling of plasmon resonances in a thin metal film and the waveguide modes in a dielectric overcoating.
2. Description of the Related Art
Surface plasmon resonance is a technique used in the development of gas sensors, measurement of optical properties of metals, degradation monitoring of metals, microscopy, and chemical and biochemical sensing. Among optical techniques such as ellipsometry, multiple internal reflection spectroscopy, and differential reflectivity, SPR is one of the most sensitive techniques to surface and interface effects. This inherent property makes SPR well suited for nondestructive studies of surfaces, interfaces, and very thin layers. SPR also has uses other than surface investigations and it has recently been demonstrated as a new optical technique for use in immunoassays.
The SPR phenomenon has been known for over 25 years and the theory is fairly well developed. Simply stated, a surface plasmon is an oscillation of free electrons that propagates along the surface of a conductor. The phenomenon of surface plasmon resonance occurs under total internal reflection conditions at the boundary between substances of different refractive indices, such as glass and water solutions. When an incident light beam is reflected internally within the first medium, its electromagnetic field produces an evanescent wave that crosses a short distance (in the order of nanometers) beyond the interface with the second medium. If a thin metal film is inserted at the interface between the two media, surface plasmon resonance occurs when the free electron clouds in the metal layer (the plasmons) absorb energy from the evanescent wave and cause a measurable drop in the intensity of the reflected light at a particular angle of incidence that depends on the refractive index of the second medium.
Typically, the conductor used for SPR spectrometry is a thin film of metal such as silver or gold; however, surface plasmons have also been excited on semiconductors. The conventional method of exciting surface plasmons is to couple the transverse-magnetic (TM) polarized energy contained in an evanescent field to the plasmon mode on a metal film. The amount of coupling, and thus the intensity of the plasmon, is determined by the incident angle of the light beam and is directly affected by the refractive indices of the materials on both sides of the metal film. By including the sample material to be measured as a layer on one side of the metallic film, changes in the refractive index of the sample material can be monitored by measuring changes in the surface plasmon coupling efficiency in the evanescent field. When changes occur in the refractive index of the sample material, the propagation of the evanescent wave and the angle of incidence producing resonance are affected. Therefore, by monitoring the angle of incidence at a given wavelength and identifying changes in the angle that causes resonance, corresponding changes in the refractive index and related properties of the material can be readily detected.
As those skilled in the field readily understand, total reflection can only occur above a particular critical incidence angle if the refractive index of the incident medium (a prism or grating) is greater than that of the emerging medium. In practice, total reflection is observed only for incidence angles within a range narrower than from the critical angle to 90 degrees because of the physical limitations inherent with the testing apparatus. Similarly, for systems operating with variable wavelengths and a given incidence angle, total reflection is also observed only for a corresponding range of wavelengths. This range of incidence angles (or wavelengths) is referred to as the "observable range" for the purpose of this disclosure. Moreover, a metal film with a very small refractive index (as small as possible) and a very large extinction coefficient (as large as possible) is required to support plasmon resonance. Accordingly, gold and silver are appropriate materials for the thin metal films used in SPR; in addition, they are very desirable because of their mechanical and chemical resistance.
Thus, once materials are selected for the prism, metal film and emerging medium that satisfy the described conditions for total reflection and plasmon resonance, the reflection of a monochromatic incident beam is a function of its angle of incidence and of the metal's refractive index, extinction coefficient, and thickness. The thickness of the film is therefore selected such that it produces observable plasmon resonance when the monochromatic light is incident at an angle within the observable range.
The classical device by which SPR is carried out is known as the Kretschmann prism arrangement, illustrated by the sensor apparatus 10 of FIG. 1. A thin film 12 of metal is coated on one face 14 of a prism 16 which has a high refractive index n (in the 1.4-1.7 range). Gold or silver films are most often used due to their refractive and extinction properties, as described above, and the relative ease with which these metals can be deposited onto a substrate with an accurate thickness. The surface chemistry of gold and its resistance to oxidation make it the prime choice for SPR experiments, although many other materials can support surface plasmon (SP) waves. As well understood by those skilled in the art, the main criterion for a material to support SP waves is that it have a negative real dielectric component, which results from the refractive and extinction properties outlined above. Although materials other than metals can support SPR, metals are most commonly used; accordingly, metals are used here to denote a support surface for SP waves.
It is noted that another type of sensor used for conventional SPR is the Otto device, which consists of the same elements illustrated in FIG. 1 but with a very thin air gap between the face 14 of the prism 10 and the metal film 12. The principles for the design and operation of these two devices are the same. Therefore, this entire disclosure is intended to refer to both types of device even though the figures illustrate only Kretschmann prism arrangements.
The surface 18 of the metal film 12 forms the transduction mechanism for the sensor 10 and is brought into contact with the sample material 20 to be sensed at the interface between the metal film and the emerging medium contained in a cell assembly 22. Monochromatic light L is emitted by a laser or equivalent light source 24 into the prism or grating 16 and reflected off the metal film 12 to an optical photodetector 26 to create the sensor output. The light L launched into the prism and coupled into the SP mode on the film 12 is p-polarized with respect to the metal surface where the reflection takes place. According to these prior-art devices and techniques, only p-polarized light is coupled into the plasmon mode because this particular polarization has the electric field vector oscillating normal to the plane that contains the metal film. This is sometimes referred to as transverse-magnetic (TM) polarization.
As mentioned, the surface plasmon is affected by changes in the dielectric value of the material in contact with the metal film. As this value changes, the conditions necessary to couple light into the plasmon mode also change. For the particular sensing system described in FIG. 1 (and for the corresponding Otto configuration), the angle of incidence .alpha. for the light beam L with respect to the metal surface 18 and the reflected light intensity are the measured parameters of interest. If the angle of incidence for the light beam is scanned throughout a range of values, a distinct minimum in reflectivity is observed at a discrete angle associated with a given refractive index in the sample material 20. This angle is commonly known as .alpha..sub.sp, the surface plasmon coupling angle. At this particular angle of incidence, set of dielectric values, and optical wavelength, the light L is being coupled into the plasmon mode and the reflection is attenuated. There is a distinct coupling angle where most of the light is attenuated for each sample material. Thus, as illustrated schematically in FIG. 2, measurements are carried out by mounting the sensor device 10 on a table 28 capable of rotating with respect to the fixed light source 24 and by relating .alpha..sub.sp to changes in the dielectric values or refractive index of the sample material 20.
SPR is a highly sensitive technique useful for investigating changes that occur at the surface 18 of the metal film. Therefore, the basic SPR sensor device 10 has been used in a wide variety of SPR research applications. In particular, over the last decade there has been a renewed interest in the application of surface plasmon resonance spectroscopy to study the optical properties of molecules immobilized at an interface between solid and liquid phases. This invention was made and is disclosed herein in this context. As described in detail in recently published articles, the ability of the SPR phenomenon to provide information on the physical properties of thin films deposited on a metal layer, including lipid and protein molecules forming proteolipid membranes, is based upon two principal characteristics of the effect. The first is the fact that the evanescent electromagnetic field generated by the free electron oscillations decays exponentially with penetration distance into an emergent dielectric medium; i.e., the depth of penetration into the dielectric material in contact with a metal layer extends only to a fraction of the light wavelength used to generate the plasmons. This makes the phenomenon sensitive to the optical properties of the metal/dielectric interface without any interference from the properties of the bulk volume of the dielectric material or any medium that is in contact with it. The second characteristic is the fact that the angular (or wavelength) position and shape of the resonance curve is very sensitive to the optical properties of both the metal film and the emergent dielectric medium adjacent to the metal surface. As a consequence of these characteristics, SPR is ideally suited for studying both structural and mass changes of thin dielectric films, including lipid membranes, lipid membrane-protein interactions, and interactions between integral membrane proteins and peripheral, water-soluble proteins. See Salamon, Z., H. A. Macleod and G. Tollin, "Surface Plasmon Resonance Spectroscopy as a Tool for Investigating the Biochemical and Biophysical Properties of Membrane Protein Systems. I: Theoretical Principles," Biochim. et Biophys. Acta, 1331: 117-129 (1997); and Salamon, Z., H. A. Macleod and G. Tollin, "Surface Plasmon Resonance Spectroscopy as a Tool for Investigating the Biochemical and Biophysical Properties of Membrane Protein Systems. II: Applications to Biological Systems," Biochim. et Biophys. Acta, 1331: 131-152 (1997).
The present invention is directed at improving these prior-art sensor devices and procedures by providing new thin-film interface designs that couple surface plasmon and waveguide excitation modes. The resulting devices, referred herein as coupled plasmon-waveguide resonators (CPWR), exhibit several new properties that constitute material advances in the art.